Both excess adipose tissue and insufficient adipose tissue result in insulin resistance and diabetes. Therefore understanding the processes by which adipocytes form is of great interest. Much of what we have learned of this process has come from 30 years of analysis of 3T3-L1 cells differentiating in vitro when given an adipogenic cocktail of dexamethasone (glucocorticoid), IBMX (raises intracellular cAMP levels) and insulin. This cocktail has been shown to induce adipogenic transcription factors such as C/EBPs and PPAR-3. These transcription factors have been shown to be required for adipogenesis in vivo in mice, but the cues that induce them in vivo during adipogenesis are unknown. There is evidence from the literature to suggest that the cues for adipogenesis in vivo may differ from those used in vitro. For example, mouse embryonic fibroblasts from lipoatrophic mice with no fat differentiate competently into lipid-laden adipocytes. It is not possible to assess the requirement for GR in adipogenesis in GR-null mice because they die at birth prior to adipogenesis in vivo. To address this problem this proposal will use transplantation of adipose stem cells in mice. Aim 1 To establish a model of adipose stem cell transplantation to study adipogenesis in vivo. An array of potential adipogenic stem cell types (from mice with the unique property of activating luciferase upon forming adipocytes) will be transplanted into host mice. Aim 2 To determine the requirement for glucocorticoids in adipogenesis in vivo. Adipogenic stem cells will be transplanted into adrenalectomized or control host mice. Adipogenesis will be assessed by bioluminescent in vivo imaging. Aim 3 To determine the role of GR in adipogenesis in vivo. Adipogenic cells that are GR-null or GR-wildtype control will be transplanted into host mice and their potential to undergo adipogenesis assessed by molecular markers and histology. These Aims should answer the question as to whether glucocorticoid ligand and receptor are required for adipogenesis in vivo.